I have to admit, I took "series" to mean franchise here, haha.

It's a Visual Novel (not the dating sim kind, just a story) by 07th expansion called Umineko No Naku Koro Ni (On steam it's called Umineko When They Cry). They also made Higurashi no naku koro ni, which is actually a lot more well known.

It's a mystery novel where the reader is challenged to figure it out on their own. Is it the thousand year old witch, Beatrice or is it one of the family members? How? Why?

More than that I can't really say about it. Even that is skirting the edge of spoiler territory. The music is absolutely phenomenal. And the story has so many layers that a reread makes it an entirely different story.

CW: It does have gore, but never outright shown, just text descriptions on a background of the same png image of blood (there's 4 variants or something?)